1. Field of the Invention
This invention relates to electrolyte membranes suitable for use in proton exchange membrane fuel cells. More particularly, this invention relates to proton exchange membranes suitable for use in proton exchange membrane fuel cells operating at temperatures ranging from about room temperature to about 170° C.
2. Description of Related Art
A fuel cell is an electrochemical device in which the chemical energy of a reaction between a fuel and an oxidant is converted directly into electricity. The basic fuel cell unit comprises an electrolyte layer in contact with a porous anode and cathode on either side. In a typical fuel cell, a gaseous or liquid fuel is continuously fed to the anode electrode, sometimes referred to as the fuel electrode, and an oxidant, such as oxygen from air, is continuously fed to the cathode electrode, sometimes referred to as the air electrode, and electrochemical reactions occur at the electrodes to produce an electric current. Due to the limited electricity generating capacity of individual fuel cell units, a plurality of fuel cell units are typically stacked one on top of another with a bipolar separator plate separating the fuel cell units between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit.
There are a number of different fuel cell types which are classified based upon a variety of categories including the combination of type of fuel and oxidant, whether the fuel is processed external to or inside the fuel cell, the type of electrolyte, e.g. solid oxides, phosphoric acid, molten carbonate and proton exchange membranes, the temperature of operation and whether the reactants are provided to the fuel cell by internal or external manifolds.
This invention relates to proton exchange membrane fuel cells, also sometimes referred to as polymer electrolyte membrane fuel cells. In a proton exchange membrane fuel cell, the electrolyte is a proton conducting membrane sandwiched between two porous electrodes. The backs of the electrodes are made hydrophobic by coating with an appropriate compound, such as TEFLON®. Proton conducting membranes conventionally used in proton exchange membrane fuel cells are made of a perfluorinated sulfonic acid polymer, an example of which is sold under the brand name NAFION® by DuPont. NAFION membranes, which are fully fluorinated polymers, have exceptionally high chemical and thermal stability and are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, Cl2, H2 and O2 at temperatures up to about 100° C. NAFION consists of a fluoropolymer backbone upon which sulfonic acid groups are chemically bonded. However, although an exceptional performer, NAFION is an expensive material and makes proton exchange membrane fuel cells economically unattractive in most applications. Much of the cost of NAFION is due to two factors: the use of fluorine and the very severe reaction conditions needed to prepare the polymer.
The proton exchange membrane fuel cell is suitable for a wide range of power generating applications including vehicular applications. The proton exchange membrane fuel cell system for automobile applications requires operation at temperatures in excess of about 100° C. to reduce the size of radiators, increase fuel efficiency and provide better water and heat management. At the present time, the most common high temperature proton exchange membranes for fuel cells are inorganic metal oxide-doped NAFION and phosphoric acid doped polybenzimidazole (PBI) and its derivatives. In the metal oxide-doped NAFION membrane, the metal oxide is used to retain water within the membrane at elevated temperatures, thereby maintaining the membrane proton conductive. In the PBI membrane, the phosphoric acid is proton conductive at 120° C. to 160° C. However, these two types of membranes do not exhibit promising performance and lifetimes under fuel cell operating conditions. In the metal oxide-doped NAFION membrane, the inorganic metal oxide leaches out over time due to the absence of a stable bond between the inorganic species and the binding membrane. Similarly, in the PBI membrane, the phosphoric acid departs from the membrane with product water in the fuel cell because the phosphoric acid and the imine group in the PBI membrane are only weakly bonded. In the proton exchange membrane fuel cell, the membrane is required to operate under a wide range of temperatures from about room temperature to about 170° C. so that the fuel cell can both cool-start and operate at high temperature conditions.
Accordingly, the challenge is to find lower cost membranes having the desired properties for use as a proton conductor in proton exchange membrane fuel cells. Some of these properties include mechanically stable and durable film behavior in the cell-operating environment with long lifetimes, hydrophilicity for high conductivity and water insolubility. Low cost membranes in the form of sulfonated polystyrene membranes have been applied to proton exchange membrane fuel cells. However, these membranes can only be used at temperatures of less than about 100° C. Membranes capable of operating at higher temperatures, about 120° C. to about 170° C., have the advantages of enhanced CO tolerance, which enables simplification of the entire fuel cell system; improved cathode reaction kinetics, which enables the use of higher stack power densities; and reduced heat exchanger requirements. However, the stability of high temperature membranes is still problematic.